The most common current practice in telephone communication systems generally is to establish a solid connection between a calling line and a called line via a path which is associated individually and uninterruptedly with the connection for the duration of the call. Thus a quantity of equipment, dependent upon the number of lines served and the expected frequency of service, is provided in a common pool from which portions may be chosen and assigned to a particular call. Such an arrangement is referred to as "space division" in which the privacy of each conversation is assured by the division or separation of individual conversations in space.
In contrast, communication systems have been developed which operate on a time division basis in which a number of conversations share a single spatial communication highway or bus. Privacy of conversation is assured in such systems by the division or separation of individual conversations in time. Thus each conversation is assigned to the common spatial highway for an extremely short, periodically recurring interval, called a time slot, and the connection between any two lines in communication is completed only during the assigned interval or time slot. Samples which retain essential characteristics of the voice or other signal are transmitted over the common highway in these time slots and are utilized in the called line to reconstruct the original signal.
Existing time division multiplex telephone systems are designed to handle a very large number of telephones. There is a need for a smaller private automatic branch exchange (PABX) having the economy, the flexibility, and the adaptability to computer control possessed by the time division multiplex (TDM) approach, particularly as an integral part of a computer-controlled property-management system including perhaps 150 telephones and apparatus for managing the business of the property, which may be a hotel, motel, hospital, school, or the like.